(1) Field of the Invention
The present invention relates to a band-pass resonator-type surface-acoustic-wave filter having a plurality of surface-acoustic-wave resonators connected in series and parallel with one another. More particularly, this invention is concerned with a resonator-type surface-acoustic-wave filter for reducing the signal height of a spurious peak that causes the extra-passband suppressibility to deteriorate and occurs especially at a frequency outside a passband.
(2) Description of the Related Art
A known high-frequency band pass filter includes a surface-acoustic-wave (SAW) resonator in which different electrode patterns are formed in reflectors and an InterDigital Transducer (IDT) on a piezoelectric element. With the recent popularity of mobile telephones and portable telephones, there is an increasing demand for SAW resonators as small, thin radio-frequency (RF) filters.
Generally, the SAW resonator element has a pair of reflectors each of which has a plurality of reflector electrodes arranged in the form of a grid on a substrate made of a piezoelectric material or a magnetostrictive material. An IDT having numerous pairs of screen-like electrodes is interposed between the two reflectors. Resonance resulting from reciprocation of a surface acoustic wave between the two reflectors is fed to an electric circuit via input/output terminals. The pitch between adjoining reflector electrodes, which is referred to as an inter-electrode pitch, in the reflectors shall be defined as .lambda..sub.REF, and the pitch between adjoining interdigital electrodes in the IDT, which is referred to as an inter-electrode pitch, shall be defined as .lambda..sub.IDT. The values of the inter-electrode pitches are determined according to a resonant frequency. In the prior art SAW resonator element, .lambda..sub.REF and .lambda..sub.IDT have an equal value.
A SAW filter that has been adopted widely to date has a transversal structure or what is referred to as a cascade structure in which transversal structures are cascaded in two stages. For example, Japanese Unexamined Patent Publication (Kokai) No.57-99813 has disclosed a transversal SAW filter, and Japanese Unexamined Patent Publication (Kokai) No.4-94208 has disclosed a cascaded SAW filter. In each of SAW filters shown in these documents, reflectors are arranged in both sides of an IDT.
A SAW resonator with a pair of terminals includes an equivalent circuit electrically equivalent to a crystal oscillator. As already known, a resonator-type filter can be realized by connecting the SAW resonators in series and parallel with one another in the form of a multistage structure. Therefore, the resonator-type filter is available in various modes dependent on connection arrangements. In a resonator-type filter based on SAW resonators, each SAW resonator having a pair of terminals is treated as a pure reactance element in order to utilize a resonant effect. Impedance occurring at any of frequencies in a passband can therefore be limited. This obviates the necessity of an external matching circuit and realizes a reduced insertion loss. The resonator-type filter acts as a narrow-band band pass filter because of the generally large capacity ratio among resonators. Owing to characteristics similar to those of a narrow-band band-pass filter, the resonator-type filter is used as an RF filter for a mobile telephone or a portable telephone.
Thus, the SAW resonator having a pair of terminals and the resonator-type filter based on the SAW resonators differ from each other even in terms of the design technology. The present invention relates to a resonator-type filter composed of SAW resonators each having a pair of terminals. Hereafter, the resonator-type filter composed of SAW resonators each having a pair of terminals is referred to as a resonator-type SAW filter.
The resonator-type SAW filter offers great freedom in specifying constants for an individual resonator in the process of design or in selecting a combination of resonators to be connected in series or parallel with one another. This leads to a low loss and a relatively large stop-band attenuation ratio. Thanks to these advantages, resonator-type SAW filters are taking the place of conventional filters.
One of the important characteristics of a band pass filter is stop-band suppressibility. The stop-band suppressibility indicates to what extent signals with frequencies outside a passband can be blocked relative to signals with frequencies within the passband. When a band pass filter is employed as an RF filter for radio communications, if the stop-band suppressibility is small, interference and other problems may occur. Therefore, a large stop-band suppressibility is usually desired.
In a conventional resonator-type SAW filter, a spike or what is referred to as a spurious peak occurs at a certain frequency outside a passband because of the structure of the filter. The spurious peak causes the stop-band suppressibility to drop or the attenuation ratio to rise like a spike. In equipment using the resonator-type SAW filter, when the frequency causing the spurious peak corresponds to a frequency that must be suppressed, the suppressibility deteriorates by a degree corresponding to the height of the spurious peak.
When a resonator-type SAW filter is needed, a resonator-type SAW filter permitting high suppressibility at all stop-band frequencies should be adopted on the assumption that stop-band suppressibility may deteriorate due to the spurious peak. In this case, occurrence of a spurious peak would pose no problem. Alternatively, a spurious peak is reduced in signal height by adjusting the combination of or the number of stages of SAW resonators each having a pair of terminals to be connected in series or parallel with one another. However, it is very hard to manufacture a resonator-type SAW filter permitting high suppressibility at all stop-band frequencies. Adjusting the combination of or the number of stages of SAW resonators each having a pair of terminals requires considerable labor and is quite time-consuming.